In a variety of manufacturing applications it is often imperative that a predetermined amount of torque be applied to a fastener to form a proper joint. For example, in automotive applications, bolts must be tightened within a certain prescribed range of torque to properly join two parts together thereby assuring good reliability of the joint during expected use. A relatively simple test has been used in the past to measure fastener torque levels. An operator uses a hand torque wrench to engage the fastener to be tested. He then uses the wrench to apply more torque to the fastener until it finally begins to rotate in the tightening direction. Early techniques called for the operator to merely view the reading of the wrench torque indicator just prior to the "give" or "breakaway" of the fastener as this torque level was thought to be generally associated with the amount of torque originally applied to the fastener during the normal assembly process. Later improvements of such a test included the use of a wrench which would maintain the position of the indicator at the maximum torque level experienced.
Unfortunately, the prior art methods of sensing the applied torque were not very precise and the results were not capable of being accurately reproduced from operator to operator. The breakaway torque level was hard to accurately measure because it was difficult for the operator to instantaneously stop applying any more torque as soon as he noticed fastener motion. Hence, the torque reading was often too high due to this overshooting problem.
U.S. Pat. No. 4,244,213 and U.S. Pat. No. 4,319,494 to Marcinkiewicz (hereby incorporated by reference) disclose dramatic improvements in retorque measuring techniques. These patents broadly disclose the concept of electronically and automatically detecting the amount of previously applied torque to a fastener. In general, electrical circuitry is used to automatically detect a change in slope of the torque signal. The torque value associated with the occurrence of the slope change is displayed as being representative of the amount of torque previously applied to the fastener. Preferably, the circuitry is adapted to detect the torque signal value associated with a negative valley occurring after the breakaway point. This negative valley torque, when it occurs, provides an even better indication of the amount of torque applied to the fastener during its original tightening process.
While the above commonly assigned patents certainly advanced the state of the art, the particular embodiments disclosed therein for carrying out their broad teachings can be even further improved. Spurious peaks or spikes in the torque signal are often encountered under true operating conditions. These spikes can be generated by things like electrical noise but generally they are due to the operator "jerking" the wrench during the test instead of smoothly applying the torque to the fastener. Unfortunately, the analog circuit approach of the previous patents cannot readily filter out those signals. Since their detection schemes look for changes in relative torque values these spikes could trigger false readings.
The present invention is directed to solving one or more of these problems.